John, Alan is on the right path.  You need an RC network, where you calculate
the worst case DI/DT.  For instance, if you have current zero cross (where
triac turns off), and voltage is 90 degrees away at max peak value, then you go
from 0V across the triac to 160/320V across the triac.  If you couple enough
electrons into the gate, it will turn back on.  Then you have an expensive
piece of wire.

The RC snubber is intended to act as a limit to the DI/DT ramp rate that the
triac is exposed to.  You can calculate a bunch of stuff, but in the end you
have to measure current, probably with a current probe.

Also, do not forget that the optocoupler is susceptable to the same phenomena.
You have to protect the triac from high DI/DT, again with an RC, with one more
R in series for the gate current limit.  This classic RCR setup is pretty
common.  There is a way to calculate all of the values, too.

Don't forget, if you want to pass any standards comittees, you should use X2
class capacitors and flameproof resistors.  Otherwise, they will give you the
hairy eyeball.

Chris~

"John Walshe @Inpact" wrote:

> Speaking of limiting the DI/DT to protect the triac - what methods are used
> to do this when (trying) to control a 1300w motor.
> John
>
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